Investigating the Three-Dimensional Microstructural Characteristics of Lithium-Sulfur Electrodes with X-Ray Micro-Tomography

Abstract

Lithium sulfur (Li-S) batteries offer a theoretical specific capacity of 1675 mAh g-1, an order of magnitude greater than current Li-ion battery technology, along with lower cost and improved safety. However, their commercialization has been hindered by poor cycle life and low coulombic efficiency. As opposed to the relatively simple ‘rocking chair’ intercalation-based mechanism underpinning Li-ion battery technology, the reaction mechanism within Li-S conversion-type cathodes involves multiple stages and phase changes. During the charge process, solid lithium sulfide is delithiated by a series of electrochemical reactions involving soluble polysulfide intermediates before being converted into elemental sulfur, and during the discharge process, the reverse occurs. The development of Li-S batteries has been hindered by complex phenomenon including large volume change (ca. 80%) between elemental sulfur (S) and lithium sulfide (Li2S), active material loss resulting from polysulfide dissolution, poor electrical conductivity of both S and Li2S, significant self-discharge, and the polysulfide shuttle mechanism causing poor coulombic efficiency and anode corrosion.Whilst recent advances have been made in contained cathode materials for Li-S batteries to prevent polysulfide dissolution into the bulk electrolyte phase and suppress active material loss and the polysulfide shuttle effect, a more comprehensive understanding of the underlying mechanisms contributing to active material loss to the electrolyte phase is essential for further optimization of cycle life and capacity, particularly so for high mass loading sulfur cathodes. However, the roles that the electrode morphology and the microstructure of the carbon binder domain (CBD) play in contributing to these failure mechanisms during cycling remain poorly understood. Lab-scale micro- and nano-tomography was performed on a Li-S battery with the ZEISS Xradia Versa 520 and Ultra 810 X-ray microscopes, achieving voxel dimensions of ca. 126 nm and 30 nm respectively. These instruments enabled a multi length-scale, three-dimensional X-ray tomography study to be carried out, with the extraction of various metrics as a function of cycle life and state of charge on the sulfur cathode. The metrics included morphological parameters, such as sulfur mass loading and particle size distribution, and microstructural characteristics, such as electrode porosity, tortuosity and contact area between phases. Nano-tomography of the CBD revealed the resolution dependent nature of certain parameters (i.e. contact area between phases), and highlighted the significance of multi length-scale characterization. Digital volume correlation (DVC) was applied to track the evolution of sulfur particles as a function of cycle number. The 3D visualization of Li-S electrodes with this technique provides a wealth of information, improving understanding of the evolution of morphological and microstructural parameters of the different phases; as X-ray imaging is inherently non-destructive, it enables these parameters to be observed in-situ as a function of cycle life and state of charge (SoC). The inherently anisotropic nature of active material dissolution, recrystallization and other degradation processes within the electrode necessitates a three-dimensional approach to enable their quantification, and X-ray tomography is a useful tool in informing the development of more optimal electrode designs. Figure 1